There exist various plausible rationales as to why Michael Jackson was not bestowed with an abundance of film roles subsequent to "The Wiz." One such rationale may be attributed to the fact that "The Wiz" failed to attain commercial triumph, despite its esteemed cast and extravagant budget.
Another feasible explanation could be that Jackson's melodic career was rapidly escalating, and consequently, he was inundated with studio recording and concert engagements.
Furthermore, it is conceivable that Jackson's physical appearance and conduct underwent a drastic metamorphosis after "The Wiz," which conceivably may have hampered his attraction as a movie star. Moreover, owing to his particularity about the roles he portrayed, Jackson might have spurned offers that were incongruent with his artistic principles.
As for the second inquiry, it is arduous to affirm with certainty that the "Thriller" album would have never seen the light of day if Jackson had pursued a career in acting. Although it is plausible that a prosperous acting career may have monopolized a significant portion of his time and efforts, Jackson's ambitious and unyielding temperament might have enabled him to concurrently pursue both ambitions.
Ultimately, conjecturing on what could have been is the only option at our disposal.